Re: [Maxima-discuss] sometimes 1//2 =/= 1/2

[Barton W. <wi...@un...>]

Another thing:  Although bfhalf automatically updates when  fpprec changes, some special variables defined in globals.lisp do not. Examples:

(%i1) :lisp(print bigfloat%pi)

((BIGFLOAT SIMP 56) 56593902016227522 2)
((BIGFLOAT SIMP 56) 56593902016227522 2)

(%i1) fpprec : 57$

(%i2) :lisp(print bigfloat%pi)

((BIGFLOAT SIMP 56) 56593902016227522 2)
((BIGFLOAT SIMP 56) 56593902016227522 2)
________________________________
From: Raymond Toy <toy...@gm...>
Sent: Thursday, December 4, 2025 5:44 PM
To: Barton Willis <wi...@un...>; max...@li... <max...@li...>
Subject: Re: [Maxima-discuss] sometimes 1//2 =/= 1/2

Caution: Non-NU Email


On 12/4/25 8:51 AM, Barton Willis wrote:

I have tested changing (let ((1//2  ... to (let ((bfhalf ....  It fixes the trouble and eliminates the need for the macro half that is used in absarg.   I hope to commit this fix along with a few others in a few weeks.

Hmm. float.lisp defines bfhalf spec var. And it's value is updated whenever fpprec1 is called. Maybe you don't need that at all.

And we should probably rename bfhalf to *bfhalf* along with the other spec vars updated in fpprec1. And rename 1//2 to *1/2*. Or make 1//2 a defconstant. But sbcl is really picky about what can be a defconstant.


________________________________
From: Raymond Toy <toy...@gm...><mailto:toy...@gm...>
Sent: Thursday, December 4, 2025 10:28 AM
To: max...@li...<mailto:max...@li...> <max...@li...><mailto:max...@li...>
Subject: Re: [Maxima-discuss] sometimes 1//2 =/= 1/2


Caution: Non-NU Email


On 12/4/25 5:43 AM, Barton Willis via Maxima-discuss wrote:

OK, maybe I found the culprit:

(defun bfloat-erf (z)
  ;; Warning!  This has round-off problems when abs(z) is very small.
  (let ((1//2 ($bfloat '((rat simp) 1 2))))
  ;; The argument is real, the result is real too
    ($realpart
      (mul
        (simplify (list '(%signum) z))
        (sub 1
          (mul
            (div 1 (power ($bfloat '$%pi) 1//2))
            (bfloat-gamma-incomplete 1//2 ($bfloat (power z 2)))))))))



And similarly for complex-bfloat-erf .  Changing  1//2 to bfhalf in this code didn't result in any testsuite failures, but I have a few pending changes to the function  absarg.
Nice bit of detective work! Yeah, renaming 1//2 to something else should take care of the problem.

Also, loading gamma.lisp gives the warning: (SBCL)

 in: DEFUN GAMMA-INCOMPLETE
;     (COND ((COMPLEXP MAXIMA::X) #C(0.0d0 0.0d0)) ((REALP MAXIMA::X) 0.0d0))

I think there are lots of these.

The issue here is that the compiler doesn't know what X is, so the cond cases of complexp and realp covers the case of numbers, but there are lots of other types which aren't covered.

This can be fixed in two ways. Add (declare (number x)). Add a T clause to cond. This should probably signal an error.

Or maybe even replace the cond with etypecase?

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